This invention relates to an irradiation system with an ion beam/charged particle beam and, in particular, relates to an energy filter for use therein.
An irradiation system with an ion beam/charged particle beam (also called an ion implantation system; hereinafter referred to simply as a “irradiation system with a beam”) is a system for applying a mass analysis to ions or charged particles extracted from a beam source to thereby select only a necessary ion species or necessary charged particles and irradiating a wafer with the beam.
Among this type of irradiation systems with the beam, there is available one having, in addition to a mass analyzer, an energy filter called an angular energy filter (AEF) in order to more accurately implant ions or charged particles into a wafer.
The angular energy filter includes an analyzing electromagnet having a hollow-portion for passing an ion beam or a charged particle beam therethrough. The angular energy filter deflects the ion beam/charged particle beam passing through the hollow-portion of the analyzing electromagnet by a Lorentz force so as to irradiate the wafer with only those ions or charged particles each having a predetermined energy.
In order to cope with an increase in diameter of wafers in recent years, the irradiation range with a beam is required to be increased and, in order to satisfy such a requirement, the size of a hollow-portion of an analyzing electromagnet tends to be enlarged. When increasing the irradiation range with the beam, its cross-section is formed into an oval or ellipse that is long in one direction while short in a direction perpendicular thereto in order to make uniform the ion/charged particle density of the beam. Alternatively, the beam is reciprocatingly deflected for scanning along one direction in order to increase the irradiation range with the beam so that the beam has an oval, elliptical, or belt-shaped continuous cross-section. Accordingly, the shape of the hollow-portion of the analyzing electromagnet is normally rectangular and magnetic poles thereof are located at short sides of the rectangle. Therefore, as the hollow-portion of the analyzing electromagnet is enlarged, a gap between the magnetic poles becomes longer so that the spread of magnetic field distribution (leakage magnetic field) increases.
The irradiation system with the beam comprises a plasma shower device for suppressing charge-up of the wafer caused by irradiation with an ion beam. However, using the energy filter including the electromagnet with the large hollow-portion, there is a possibility that its leakage magnetic field gives bad influence to the wafer neutralization effect of charge-up suppression plasma shower electrons. Therefore, the leakage magnetic field of the energy filter should be cancelled.
Further, an implantation angle (irradiation angle) of the ion beam/charged particle beam with respect to the wafer should be constant regardless of an irradiation position with the beam. Specifically, the analyzing electromagnet is required to form a uniform magnetic field (where the BL product is constant. BL product, which is BL integral, a integrated magnetic flux density along the beam path.) on a path of the ion beam or charged particle beam so that the beam is deflected at the same angle wherever the beam passes within the hollow-portion of the analyzing electromagnet.
In a conventional energy filter using an analyzing electromagnet of which a gap between magnetic poles is narrow, using a field clamp for canceling a leakage magnetic field.
There is another conventional energy filter that uses a superconductive material for suppressing a leakage magnetic field. Such an energy filter is disclosed in, for example, JP-A-H06-111759.
On the other hand, as a conventional method of making uniform a magnetic field distribution produced in an opening (or hollow-portion) of an analyzing electromagnet, there is a method of winding a main coil at the center of a yoke and auxiliary coils on both sides thereof and adjusting a magnetic field generated by the main coil by the use of the auxiliary coils. Such a method is described in, for example, JP-A-H10-302706 or JP-A-H10-302707.
However, there is a problem that although the conventional method of suppressing the leakage magnetic field of the analyzing electromagnet by the use of the field clamp is suitable for the analyzing electromagnet having a small opening (or hollow-portion) with the narrow magnetic gap, it is insufficient for suppressing a leakage magnetic field from an analyzing electromagnet having a large opening (or hollow-portion).
Further, there is a problem that the conventional method of suppressing the leakage magnetic field by the use of the superconductive material is applied to a C-shaped magnet but is not applicable to an analyzing electromagnet having a hollow-portion at its center. In addition, there is also a problem that such a superconductive material suppresses a leakage magnetic field leaking to both inner and outer sides in beam bending radial directions but cannot suppress a leakage magnetic field in a longitudinal direction, i.e. a leakage magnetic field directed toward a wafer.
Moreover, there is a problem that the conventional method of uniforming the magnetic field distribution aims at only the magnetic field distribution in the opening (or hollow-portion) of the analyzing electromagnet but takes into account nothing about a leakage magnetic field.